1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer, and more particularly to a polishing apparatus having a workpiece dislodgment detector for detecting a workpiece which is dislodged from a top ring while the workpiece is being polished.
2. Description of the Prior Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. Conventionally, as apparatuses for planarizing semiconductor wafers, there have been used a self-planarizing CVD apparatus, an etching apparatus or the like, however, these apparatuses fail to fully planarize semiconductor wafers. Recently, attempts have been made to use polishing apparatus for planarizing semiconductor wafers to a flatter finish with more ease than those conventional planarizing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated. This process is called Chemical Mechanical polishing.
If a semiconductor wafer is transferred normally to the top ring, then the semiconductor wafer will not be dislodged or disengage from the top ring while the semiconductor wafer is being polished. However, if a semiconductor wafer is not transferred normally to the top ring, but is partially placed on a retainer ring of the top ring, the semiconductor wafer will tend to be dislodged from the top ring while the semiconductor wafer is being polished.
When the semiconductor wafer is dislodged, i.e., slipped out from the top ring, the semiconductor wafer is rotated with the turntable and collides with the top ring, with the result that the semiconductor wafer is liable to be damaged and/or to damage components of the polishing apparatus including the retainer ring for retaining the wafer therein, the polishing cloth on the turntable, a backing pad mounted on the lower surface of the top ring and supporting the semiconductor wafer, and a dressing tool mounted on a dressing apparatus. It has heretofore been customary to detect whether there is foreign matter on the polishing cloth by direct visual observation of the surface of the polishing cloth or image processing of the surface of the polishing cloth obtained by a television camera installed over the polishing cloth or the like.
The visual observation of the surface of the polishing cloth with the naked eye or the television camera fails to accurately detect foreign matter or a semiconductor wafer on the polishing cloth depending on the property of the semiconductor wafer, e.g., whether the detected surface thereof is polished to a mirror finish or a rough finish or carries an oxide layer, and also depending on whether there is water or an abrasive liquid on the polishing cloth, or the type of an abrasive liquid present on the polishing cloth.
If the surface of the polishing cloth is monitored with a monitoring device such as a television camera, then an output signal from the monitoring device is processed by a complex process such as an image processing to detect whether or not there is foreign matter on the polishing cloth, or whether or not the polishing cloth is defective. Because the complex process is time-consuming, it takes too much time after a semiconductor wafer is dislodged until the trouble is actually detected. Even if the polishing apparatus is shut down when the malfunction is actually detected, the semiconductor wafer will be damaged or broken, and damage to components of the polishing apparatus including th polishing cloth, the backing pad and the dressing tool can occur by the time the rotations of the turntable and the top ring are stopped.
Another problem with the image processing system is that since it needs a microcomputer capable of effecting high-speed computation for processing image information from the monitoring device at a high speed, the entire polishing apparatus is complex in structure and expensive to manufacture.